Fix problem with pip 21.3 that prevents installation from source

[zklaus]

Description

This addresses an upstream bug in pip by pinning to avoid the buggy version. Compare with the sibling PR in ESMValCore at ESMValGroup/ESMValCore#1359.

• Closes With pip 21.3, pip install -e doesn't work #2350

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[valeriupredoi]

seems like conda is unable to solve the environment in practical time (I reran the test just to make sure it wasn't a fluke). Maybe we should call mamba instead or put the pin in setup.py as install requirement?

[valeriupredoi]

I was silly when I said we should pin it in setup.py - it's the old chicken and egg problem - putting it in there would mean the pip that runs the install doesn't know about it, and goes ahead with the setup, only to fail since it's 21.3 and it poops the bed. The pip that runs the install can be throttled from project.toml and as it is now, it needs to look for an alternative to 21.3, and it indeed does so: when running pip install -e .[develop] -vv:

  Running command /home/valeriu/miniconda3-fresh/envs/esmvaltool-stock/bin/python3.9 /tmp/pip-standalone-pip-6dsry2mg/__env_pip__.zip/pip install --ignore-installed --no-user --prefix /tmp/pip-build-env-f0lqpnit/overlay --no-warn-script-location -v --no-binary :none: --only-binary :none: -i https://pypi.org/simple -- 'pip != 21.3' 'setuptools >= 40.6.0' wheel
  Using pip 21.3 from /tmp/pip-standalone-pip-6dsry2mg/__env_pip__.zip/pip (python 3.9)
  Collecting pip!=21.3
    Using cached pip-21.2.4-py3-none-any.whl (1.6 MB)
  Collecting setuptools>=40.6.0
    Using cached setuptools-58.2.0-py3-none-any.whl (946 kB)
  Collecting wheel
    Using cached wheel-0.37.0-py2.py3-none-any.whl (35 kB)

[zklaus]

Sorry V, I am overriding your suggestions here. Conda should get us the right pip.

[valeriupredoi]

sure, I agree, unlike with conda, using mamba solves the env with pip restricted in environment.yml in a matter of seconds (just tried now), but I think installation dependencies should stay in pyproject.toml - although am not 100% sure of that, so I'll leave it to you 👍